Allow custom labels on PrometheusRules

[sebhoss]

Is your feature request related to a problem? Please describe.

We have multiple Prometheus instances running and want to use labels to associate PrometheusRule resources to a specific Prometheus instances. Right now, we can define those additional labels on ServiceMonitor resources but not on PrometheusRule resources.

Describe the solution you'd like

We would like to define additional labels for PrometheusRule resources, similar to how we can already do the same for ServiceMonitor resources.

Describe alternatives you've considered

We currently generate the PrometheusRule resources once, extract them, modify them by hand, apply the modified version to our cluster, and finally remove the operator managed PrometheusRule resources.